Some embodiments are directed to a displacement device. Particular non-limiting embodiments provide displacement devices for use in the semiconductor fabrication industry.
Many industrial applications may require positioning an object accurately along at least two directions which are generally orthogonal to one another. Take semi-conductor industry for example, fabrication of integrated circuits may require accurate positioning an object (e.g. a wafer or a reticle) in at least two directions for the purposes of photolithography, inspection, cutting, packaging, etc. Conventional technology stacks stages, wherein each stage may only cause a single degree-of-freedom movement, to produce combined multiple degree-of-freedom movements. For example, the well-known ‘H-bridge’ design, which includes an X-stage and a Y-stage, wherein, the X-stage is used to cause movement of the Y-stage along the X-direction and the Y-stage is used to carry an object and cause movements along the Y-direction. Combined effort of the X-stage and the Y-stage is able to position the object on the whole X-Y plane.
Recently, displacement devices were introduced that directly cause movement of the stage on at least two orthogonal directions, e.g. the X and Y directions. Such a device is generally referred to as ‘planar motor’. For some planar motors, the stage may be controlled to move in six degrees-of-freedom. A planar motor has two parts: a stator and a stage. The stator generally has larger dimension in the X-direction and the Y-direction than the stage for the purpose of providing a working region. One of the two parts includes a system of magnets and another part includes a system of electric coils. The interactions between current-carrying coils and magnets may cause movements of the stage relative to the stator. As these interactions are of electromagnetic nature, the two parts do not need any mechanical contact which eliminates disturbance forces, such as friction. As a result, higher positioning accuracy than conventional technologies may be achieved. Positioning accuracy is the key performance criteria of a displacement device. Higher positioning accuracy indicates less error between the true position of the object being positioned and the reference position. Due to the compact design, the mass of the stage is much less than that of conventional technologies. As such, the same acceleration with less force may be achieved. Compared with conventional technologies, other advantages of the ‘planar motor’ technology are easier assembling, less maintenance, suitability to use in vacuum or high-cleanness environments.
U.S. Pat. No. 6,496,093 describes a moving-coil planar motor, wherein, magnets are fixed to the stator and coils are fixed to the stage. The magnets are arranged in a pattern of rows and columns, which have 45 degrees angle difference from the orientation of the coils. This 45 degrees angle makes it difficult to assemble the magnets on the stator and to align the coils relative to the magnets. Error in alignment of coils relative to magnets compromises positioning accuracy. Furthermore, the manufacturing cost is high because a large number of magnets may be required.
International publication WO 2009/083889 describes a moving-magnet planar motor, wherein, magnets are fixed to the stage and coils are fixed to the stator. The magnets are grouped in to two kinds of magnet blocks: the first kind including magnets elongated in the X-direction and the second kind including magnets elongated in the Y-direction. The magnet blocks of the two kinds are arranged in a pattern of rows and columns with the rows parallel to the X-direction and the columns parallel to the Y-direction. The coils are formed by coil traces of two kinds: the coil traces of the first kind extend continuously over the whole stator in the X-direction and the coil traces of the second kind extend continuously over the whole stator in the Y-direction. The coil traces are arranged in multiple layers with each layer formed by coil traces of a single kind (either the first kind or the second kind).
The X-dimension and Y-dimension of the stator must or should be large enough to achieve the displacement range of the stage. If the coils are fixed to the stator, as described in international publication WO 2009/083889, the stator must or should include a large number of coils and all of them must or should be powered. This introduces a problem: either a large number of power amplifiers may be required, which indicates high cost, or the coils in the stator must or should be switched on and off according to the position of the stage (in order to reduce the number of power amplifiers), which indicates high complexity and low reliability. It is also mentioned in the international publication WO 2009/083889 that alternatively the coils may be fixed to the stage and the magnets may be fixed to the stator. However, this would end up with a planar motor with many problems. First, applying multi-phase commutation, Lorentz forces exerted on the coil traces vary with stage positions relative to the stator (position-dependent). Variation of the Lorentz forces has equivalent effect of disturbance forces, which would lower the positioning accuracy. Second, these position-dependent Lorentz forces are not able to produce a torque in a certain direction at some stage positions relative to the stator regardless of the amount of current supplied to the coils. At these positions, the rotational movement in the particular direction of the stage is uncontrollable and therefore, additional bearings (e.g. mechanical bearings, air bearings, etc.) may be required to limit the rotational movements of the stage. These bearings are undesired because they would introduce disturbances which further lower the positioning accuracy.